Rotating stratifier

ABSTRACT

An apparatus for separating a mixture of particulate materials of different densities comprising means for causing the mixture of material to flow in a circular path and means for generating a substantially radially inward air flow through said path. The circular path may be formed by an air-permeable surface of circular cross-section, for example, cylindrical or frustoconical, and the means for generating the radially inward air flow may be an annular chamber surrounding said surface in communication with a source of compressed air.

United States Patent 1191 1 1 3,844,414 Jordison Oct. 29, 1974 [54] ROTATING STRATIFIER 3,042,202 7 1962 Work 209 144 1 1 Inventor: Fred Jordison, Chesterfield, 3:331:13; 3/1323 ZLZ$fi2r'::::;,. 3131553113139 England 3,674,144 7/1972 Muller et al. 209/475 x [73] Assignee: Birtley Engineering Limited, FOREIGN PATENTS OR APPLICATIONS Darlmshlrer England 645,135 10/1950 Great Britain 209 144 [22] Filed: Apr. 17, 1972 Primary ExaminerFrank W. Lutter [21] APPL N05 244,759 Assistant ExaminerRaIph J. Hill Attorney, Agent, or Firm-Norris & Bateman [30] Foreign Application Priority Data Apr. 20, 1971 Great Britain 10125/71 [57] ABSTRACT An apparatus for separating a mixture of particulate 52 US. Cl 209/467, 209/466, 209/468, materials of different densities Comprising means for 2()9/469, 209/144 causing the mixture of material to flow in a circular 511 rm. (:1 B07b 4/08 P and means for generating a substantially radially 581 Field 6: Search 209/133, 144, 132, 152, inward air flow through said p The circular p 209 4 4 9 473475 2 2 0 512 may be formed by an air-permeable surface of circular cross-section, for example, cylindrical or frusto- 5 References Cited conical, and the means for generating the radially in- UNITED STATES PATENTS ward air flow may be an annular chamber surrounding said surface in communication with a source of com- 1,675,941 7/1928 L1ndsay.... 209/144 X pressed ain 2,643,769 6/1953 Tanner.... 209/143 2,999,593 9/1961 Stern 209/144 21 Claims, 10 Drawing Figures PATENTED HCTZS I974 3 84.4414

SHEET 2 OF 9 PATENTEDums ISM sum 30F 9 3844A PAIENTED UCTZQ I974 SHEN u 9 PATENTEUBETZQ IBM 3; 844.414 SHEET 7 OF 9 FIG] This invention relates to apparatus for separating or classifyingmixtures of particulate materials and in particular to apparatus in which a flow of air through a mixture of materials is utilised to separate the less dense material from the more dense material.

Several known apparatuses have utilised air flow through a mixture of two particulate materials of different density to separate the less dense from the more dense material. For example, in one known apparatus air is passed vertically upward through a permeable inclined deck on to which the mixture of material is fed, the deck being jigged, i.e., oscillated somewhat vertically, whilst the air is passed therethrough. The lighter material is floated by the air flow and rises to the surface of the material-on the deck and, supported by the air flow, travels down the inclined deck to the bottom thereof where it is discharged. The heavier, unfloated material is jigged up the inclined deck and is discharged at the top thereof. A contraflow of material is therefore obtained with the lighter material flowing down the deck and the heavier material flowing up the deck.

According to the present invention apparatus for the separation of a mixture of particulate materials of different densities comprises means for causing the mixture of particulate materials to move in a circular path and means for generating an air flow substantially radially inwards through said circular path.

The air flow through the mixture of particulate materials is preferably a pulsating air flow. The pulsating air flow may be generated by a pulsator air blower or'may be derived from a constant pressure source of compressed air by the use of a mechanical interrupter in the air feed passageway or duct.

The mixture of particulate materials is preferably caused to flow round the interior of an air-permeable surface of circular cross-section, for example a cylin' drical or frusto-conical surface, the radially inward air flow passing through said air-permeable surface. When the surface is of frusto-conical shape it may be axially oscillated to enhance the separation of the materials.

The mixture of particulate materials may be caused to flow round the generally cylindrical or frusto-conical surface by rotating said surface. Alternatively the mixture may be entrained in a carrier fluid which is caused to flow round a stationary surface in the manner of a cyclone.

We prefer to utilise a frusto-conical air-permeable surface and to thereby obtain a contraflow of the lighter and heavier fractions of the mixture. The mixture when fed to the interior of the frusto-conical surface is caused to flow in a circular path by rotation of the surface. The radially inward flow of air through the surface and through the mixture of materials is arranged to be such that it results in the stratification of the mixture with the lighter material disposed radially inward of the heavier material. The lighter material will travel down the slope of the frusto-conical surface whilst the heaviermaterial which is unfloated by the radially inward air flow travels up the slope under the action of the centrifugal force acting thereon, preferably aided by axial oscillation of the surface. The frusto-conical surface will preferably be disposed with its axis vertical and with its wider diameter end uppermost, its narrower diameter end being closed by a base. Thelighter material travelling down the slope of the surface towards the base may be removed by ejection through apertures in the lower portion of the surface or by suction means disposed at or near the base. The heavier material may be discharged over the upper lip of the frustoconical surface into a suitably disposed chamber or discharge shute. Whilst it is feasible to obtain the contraflow of material with a frusto-conical surface which is rotated but not axially oscillated, its capacity and eff ciency would be inferior to that in which axial oscillation were utilised.

' An apparatus according to the invention need not essentially embody contraflow separation of the heavier and lighter materials and, for example, a rotating cylindrical or frusto-conical air permeable surface may be utilised with the mixture of materials being fed in at one end and being stratified during its flow therethrough by the radially inward air flow. The strata of lighter material may be separated from the strata of heavier material, the lighter material being disposed radially inward of the heavier material, at the discharge end of the surface by a suitably disposed splitter ring or peeler knife.

The permeable surfaces of the apparatus according to the invention may be fabricated from a plurality of spaced wedge wires in the manner commonly employed in the manufacture of baskets for coal dewatering centrifuges, an example of such a centrifuge basket being described in British Patent No. 1,041,304. Alternatively the surface may be formed by expanded metal, sheet metal with punched apertures, porous ceramic, porous metal or any material of sufficient durability and porosity.

Embodiments of the invention willl now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a vertical cross-section through an apparatus being a first embodiment of the invention.

FIG. 2 is a vertical cross-section through an apparatus being a second embodiment of the invention,

FIG. 3 is a vertical cross-section through an apparatus being a third, preferred embodiment of the invention,

FIG. 4 is a part vertical cross-section through an apparatus being a modification of the third embodiment and having an alternative air feed arrangement.

FIG. 5 is a vertical cross-section throughan apparatus being a fourth embodiment of the invention,

FIG. 6 is a cross-section through an exemplary drive unit suitable for use with the apparatuses shown in FIGS. 1, 2, 3, 4, and 5,

FIG. 7 is a vertical cross-section through an apparatus being a fifth embodiment of the invention,

FIG. 8 is a cross-section through an air pulsator suitable for use with apparatus according to the invention,

FIG. 9 is a cross-section through an alternative air pulsator suitable for use with apparatus according to the invention, and

FIG. 10 is a cross-section along the lineB-B of FIG. 9

both rotary movement and axial oscillation to the basket. The drive unit 2 may be of any suitable type and conveniently is of the type conventionally used to drive the so-called vibratory centrifuges. An exemplary drive unit is shown in FIG. 6 and is hereinafter described.

The drive unit 2 is partly enclosed within a stationary cylindrical casing 3 which has an annulus 4 extending outwardly therefrom, a substantially cylindrical wall 5 extending upwardly from the outer extremity of the annulus 4 and thereby forming an annular chamber 6 surrounding the basket I. A resilient sealing strip 8 se cured to internal flange 9 formed at the top of wall 5 circumferentially contacts the basket 1 at or near the top thereof and a resilient sealing strip 10 secured to annulus 11 at the bottom of the basket 1 is held in circumferential sealing contact with the casing 3 by dish shaped member 63. Compressed air admitted to chamber 6 through pipe 7 is prevented by the sealing strips 8 and 10 from leaving said chamber other than through the porous wall of the basket 1. The chain arrows indicate the air flow.

A hopper 12 disposed above the basket 1 has an annular chute 13 adapted to feed the mixture of particulate materials from the hopper to the interior of the basket 1. An extraction duct 15 which is connected by a pipe 16 to a suction chamber and material collection point (not shown) is adapted to remove material lying on the base 14 of the basket 1. A baffle 17 is disposed within the duct 15 and encases the upper part of the drive unit 2, said baffle rendering the inlet 18 at the bottom of the duct 15 of annular shape.

An outer cylindrical wall 19 encloses the apparatus and forms a further annular chamber 20 surrounding chamber 6, said chamber 20 having an outlet 21. A lid 22 is secured to the wall 19 and closes the upper part of the apparatus. This first embodiment of the invention operates in the following manner. The mixture of more dense and less dense particulate materials is fed from the hopper 12 through the annular chute 13 to the interior of the basket 1 at a controlled rate. The centrifugal force on the mixture of particulate materials generated by the rotation of the basket 1 causes said mixture to form a generally annular layer on the interior frusto-conical surface of the basket.

Compressed air is fed through the pipe 7 to the chamber 6 and flows through the porous frusto-conical surface of the basket 1 and therefore radially inward through the rotating, generally annular layer of the material within the basket. We generally prefer to use a pulsating air flow which may be achieved either by the use of a pulsating air blower or by the use of a constant pressure source of compressed air and a mechanical interrupter in pipe 7.

The radially inward flow of compressed air through the material within the basket results in the Stratification thereof, with the strata of lighter material being disposed radially inward of the strata of heavier material. The lighter material, being floated by the air flow, travels downwardly towards the base 14 of the basket 1 from which it is extracted through the annular outlet 18 in the duct 15. In FIG. 1 the broken arrows indicate the flow of the lighter material.

The heavier material, however, is not floated by the radially inward air flow and is maintained substantially in contact with the frusto-conical surface of the basket 1 and travels upwardly over said surface aided by the jigging motion generated by the axial oscillation of the basket. On reaching the upper rim of the basket 1 the heavier material is discharged over the shedding ring affixed to said rim into chamber 20 and is discharged from said chamber through outlet 21. In FIG. 1 the solid arrows indicate the flow of the heavier material.

Thus the apparatus achieves the separation of the heavier and lighter fractions of. the mixture of materials. It will be understood that the operating conditions of the apparatus, that is the speed of revolution of the basket, the amplitude and frequency of the axial oscillation of the basket and in particular the rate of radial air flow through the mixture will have to be selected in accordance with the characteristics and nature of the component materials of the mixture in order to obtain the desired stratification of the mixture and thereby the separation of said components.

The second embodiment of the invention shown in FIG. 2 utilises a frusto-conical air-permeable basket 23 driven in rotary and axially oscillating manner by a drive unit 24 in a similar manner to the first embodiment. A frusto-conical wall 25 is secured to the basket 23 by outwardly extending axially spaced annuli 26 and 27, annulus 26 also forming the shedding ring at the rim of the basket 23 as will be hereinafter described, and thus forms an annular chamber 28 surrounding the basket 23.

Compressed air is fed from a central feed point 29 to the chamber 28 by a central rotating distributor 30 which has radially extending pipes 31 which enter the chamber 28. The chamber 29 may be provided with a mechanical interrupter 82 which renders the air flow through chamber 29 of a pulsating nature. The interrupter 82 is shown in detail in FIGS. 9 and 10 and is described hereinafter.

It will be apparent that, in contradistinction to the first embodiment of the invention, the chamber surrounding the basket and the air feed to said chamber rotate with the basket, obviating the need for the sealing strips 8 and 10 of the first embodiment.

A cylindrical wall 32 surrounds the basket and forms an annular chamber 33 which acts as the outlet for the lighter material as will be hereinafter described. A further cylindrical wall 34 forms an outer casing of the apparatus and also defines an annular chamber 36 which acts as the outlet for the heavier material as will be hereinafter described.

During operation of the apparatus the mixture of materials is fed into the basket 23 through chute 37, the basket is rotated and axially oscillated by the drive unit 24 and compressed air, preferably pulsating, is fed to chamber 28 by the distributor 30 and passes through the porous frusto-conical wall of the basket 23. The

second embodiment operates, therefore, in essentially the same way as the first embodiment with the lighter material moving downwardly as indicated by the broken arrows, and the heavier material moving upwardly, as indicated by the solid arrows. Also as in the case of the first embodiment the heavier material is discharged over the upper rim of the basket, over the shedding ring 26 into an annular chamber, that is chamber 36, from which it is discharged through outlet 38. The lighter material, however, on reaching the base 39 of the basket is not removed by suction means as in the case of the first embodiment but is ejected by centrifugal force through openings 40 in the lowermost part of the frusto-conical wall of the basket into chamber 33. When the lighter material contains a proportion of very fine particle size which it is desired to remove from the bulk of the lighter material this may be achieved by providing the chamber 33 with air extraction pipe 41 to carry away said proportion of very fine particle size.

The third embodiment of the invention shown in FIG. 3 represents our preferred embodiment of the invention. As in the caseof the first and second embodiments of the invention, an air-permeable frusto-conical basket 42 is utilised. The air-permeable surface of the basket 42 is supported by longitudinal and circumferential stiffening ribs 45 and 46 respectively. A frustoconical wall 43 is radially spaced from the basket and forms an annular chamber 44 surrounding said basket, the chamber 44 communicating via axial pipes 47 and radial passages 48 with inverted frusto-conical housing 49 which surrounds the upper part of the drive unit 50. A compressed air supply pipe 51, which has an air pulsator 52, shown in greater detail in H6. 8, is entered in the housing 49 and a gas-tight seal 53' is disposed between the stationary pipe 51 and the housing 49, which rotates with the basket 42. Pulsating compressed air supplied through pipe 51 flows through the housing 49, the radial passages 48 and the axial pipes 47 to the interior of the chamber 44 from which it flows radially inward through the permeable wall of the basket 42.

The drive unit imparts both rotary and axial oscillation movement tg the basket 4 2, t hg ro ta ry move; ment being derived from motor 53, which is, for example, hydraulic motor, which drives the large pulley 54 of the drive unit through belt drive and the axial oscillation is derived from motor 56 which is connected to the drive unit by belt drive 57 and shaft 58.1 The pulley 54 is connected to the basket by resilient members 83 which allow the basket to oscillate axially relative to the pulley. A drive unit suitable for use' with this embodiment of the invention is shown in greater detail in FIG. 6.

An. annular chamber 59 surrounding the rotary basket 42 is formed by concentric cylindrical walls 60 and 61, said chamber 59 being provided with discharge outlet chute 62. The wall 61 is the outer wall ofthe apparatus and is closed by a lid 84 which has a funnel 64 through which the mixture of particulate materials is fed to the interior of the basket 42. Baffles 65 and 66, secured to the pipe 51 and the housing 49 respectively, direct the material mixture entering through the funnel 64 on to the frusto-conical wall of the basket 42.

The third embodiment operates in a similar way to the second embodiment of the invention and the flow of the lighter material is indicated by the broken arrows and the flow of the heavier material is indicated by the solid arrows. The heavier material travels up the wall of the basket and is discharged over the upper rim and over the shedding ring 67 into the chamber 59 and is discharged therefrom through outlet chute 62. The light material travels downwardly being floated by the radially inward air flow and on reaching the lowermost part of the basket 42 is discharged through apertures at the base of the frusto-conical wall of the basket into an open bottomed chamber 69 formed by frusto-conical wall 70, a dust extraction pipe 71 communicating with said chamber 69.

The third embodiment may be modified by the inclusion of intermittently operating valves, which valves may be rotary, cam or solenoid actuated, in the axial pipes 47. These valves render the flow of air through LII LII

the pipes 47 pulsatory and, therefore, obviate the need for the air pulsator 52 in pipe 51.

In the modified air feed system shown in FIG. 4 a stationary annular air feed chamber 187 is situated below the rotating chamber 44. A plurality of pipes 68 (only one of which is shown) are slidably entered in the chamber 87 and each is urged upwardly by a spring acting on a flange 86 thereof. A sealing member 121 maintains a gas-tight seal between the flange 86 and the base 122 of the chamber 44. The base 122 is provided with a plurality of holes 123 which are so disposed as to move into and out of register with the pipes 68 as the basket 42 rotates and so a steady flow of air delivered to the chamber 87 through a pipe 124 flows intermittently into chamber 44 such that the air flow radially inward through the basket 42 is pulsatory.

Whilst the above referred to first, second and third embodiments of the invention have been described as operating in such a manner as to obtain a contraflow of the heavier and lighter fractions of the particulate material, it is possible to vary the operating conditions, i.e., the speed of revolution of the basket, the amplitude and frequency of the axial oscillation and the rate of the radially inward air flow, such that both fractions flow in the same direction. For example, FIG. 5 shows the third embodiment of the invention with the air feed system illustrated in FIG. 4 modified to operate in this alternative fashion, the basket 42 being disposed with its axis horizontal. The particulate material is fed to the interior of basket 42 and travels outwardly, that is to the wider diameter end of the basket, along the wall thereof being Stratified by the radially inward air flow, the broken arrows indicating the flow of lighter material and the solid arrows the flow of the heavier material. A cylindrical stripper or peeller blade 88 is disposed at the wider diameter end of the basket 42 and separates the heavier and lighter strata such that the lighter material is extracted through circular chute 89 and outlet 90 and the heavier material passes over shedding ring 67 to annular chamber 91 from which it is extracted through outlet 92.

FIG. 6 shows a drive unit suitable for use with the above described embodiments of the invention. The main housing 93 has located in a central bore 94 thereof a reciprocable connecting rod 95. The connecting rod 95 is located on eccentric journal 96 of shaft 58 by bearings 97, the shaft 58 being rotatably mounted in housing 93 by bearings 98. A rotor 99 rotatably mounted on housing 93 by bearings 100 and 101 has secured thereto a pulley 54 which is driven by belt drive 55. The base 102 of a basket is secured to a head 103 which is rotatably mounted on connecting rod head 104 by bearing 105. An annular resilient member is disposed between a cap 106 and the head 103, the cap 106 being secured to a cylindrical member 107 fast with the rotor 99 by rods 108 passing through bores 109 in the resilient member 110. The main rotational drive for basket is the rotor 99 which is secured to the basket by means which may be telescopic or resilient to allow vertical movement of the basket relative to the rotor 99. The axial oscillation of the basket is generated by the reciprocation of the connecting rod 95 and the head 103 secured thereto. This knowndrive unit is shown purely by way of example and it will be apparent that there are other ways of providing a drive unit with the necessary rotary and axial oscillation movements suitable for use with the apparatus according to the invention.

The fifth embodiment of the invention shown in FIG. 7 has a stationary frusto-conical air-permeable surface 111 which is surrounded by a chamber 112, air being fed to said chamber through a pipe 113. The particulate mixture is entrained in a high velocity air stream which is admitted through tangential inlet in the lid 114 which closes the upper, wider diameter end of the frusto-conical surface 111. The high velocity air stream flows spirally round the surface 111 and the radially inward air flow through the surface 111 stratifies the particulate material entrained within the high velocity air stream with the lighter fraction being disposed radially inward of the heavier fraction. An extraction fan 115 draws the lighter material through a vortex finder 116, the lighter material being collected in a dust extraction and settling chamber 117 and is discharged therefrom through chute 118. Alternatively a cylindrical splitter or peeler knife may be disposed at the lower end of the frusto-conical surface 111 such that the lighter material flows through said cylindrical splitter and the heaver material flows through the annular space between the splitter and the frusto-conical surface.

It will be apparent that the first, second, third and fourth embodiments of the invention are similar to each other and have the common feature that the material is caused to rotate in a circular path by the rotation of an air-permeable basket whereas the fifth embodiment utilises a stationary air-permeable surface and the material mixture is entrained in-a fluid caused to flow round the interior of said surface. In general we prefer to use the type of apparatus as exemplified by the first, second, third and fourth embodiments for heavy duty applications in which the material mixture is of a very coarse particle size, for example, a mixture of coal and shale, coal being the lighter material, and to use the type of apparatus exemplified by the fifth embodiment for the separation of material mixtures of finer particulate nature.

As previously mentioned we prefer to use a pulsating radially inward air flow through the mixture of particulate materials and in H6. 8 is shown a device suitable for generating such a pulsating air flow. The air pulsator 52 is situated in an air supply pipe 51, and has a radial division 72 which is provided with apertures 73. A rotor 74 mounted on the shaft 75 of a hydraulic motor 76 is provided with apertures 77 which are so disposed that they register periodically with the apertures 73 as the rotor rotates, the end of the shaft 75 remote from the'motor 76 is supported by a bearing 78 located in a hub 79 held by arms 80. The pipe 51 is connected to a source of compressed air at a constant pressure and the air flow through the pulsator is in the direction of the arrows. The apertures 73 are opened and closed by the rotor 74 as it rotates and this creates a pulsating air supply downstream of the pulsator.

In the alternative pulsator shown in FIGS. 9 and 10 there are provided three rotary vanes 81 which are rotated at the same speed and in the directions indicated by the arrows by planetary gear 119 driven by motor 120. The vanes are so synchronised that they effectively close the air supply pipe twice per revolution of each vane, thus generating a pulsating air flow downstream thereof.

1 claim:

1. An apparatus for the separation of a mixture of particulate materials of different densities comprising means defining an air-permeable frusto-conical surface having a substantially vertical axis, means for feeding a mixture of said materials to said surface, means for rotating said surface about said axis for moving the mixture of particulate materials in a path defined by said air-permeable surface, means for generating an air flow substantially radially inward through said path for effective separation of said mixture into heavier and lighter fraction containing strata, and means for separately discharging said separated materials.

2. An apparatus according to claim 1 having means to axially oscillate said surface.

3. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and said mixture is disposed in a substantially annular layer at said surface and the means for generating said substantially radially inward air flow comprises a chamber surrounding said member externally of said surface and communicating with a source of compressed air, said chamber being secured to and rotatable with the said member.

4. An apparatus according to claim 3 including means whereby compressed air is fed to said chamber surrounding the member through radial passages from a centrally disposed air feed conduit.

5. An apparatus according to claim 3 including means whereby compressed air is fed to said chamber surrounding the member through axial passages from an annular chamber in communication with a source of compressed air.

6. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and a discharge outlet for the heavier fractions is formed by circumferential apertures along one edge of said member.

7. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and a discharge outlet for the lighter fractions communicates with suction means to remove separated lighter fractions from the interior of said member.

8. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and first and second discharge outlets for the respective fractions are disposed at opposite axial ends of said member.

9. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and first and second discharge outlets for the respective fractions are disposed at the same axial end of said member.

10. An apparatus according to claim 1 wherein said means for generating the air flow comprises means producing a pulsating air flow.

11. An apparatus according to claim 10, wherein said means for producing a pulsating air flow comprises a mechanical interrupter disposed in an air feed duct communicating with a source of constant pressure air.

12. Apparatus according to claim 1 wherein the airpermeable surface is formed by a plurality of spaced wedge wires.

13. Apparatus according to claim 1 wherein the airpermeable surface is formed by expanded metal sheet.

14. Apparatus according to claim 1 wherein the airpermeable surface is formed by sheet metal with punched apertures therein.

15. Apparatus according to claim 1 wherein the airpermeable surface is formed by porous ceramic.

16. Apparatus according to claim 1 wherein the airpermeable surface is formed by porous metal.

17. In the apparatus defined in claim 1, said means defining said surface comprising a member having an air-permeable annular wall, said means for generating said air flow comprising means defining an annular chamber secured around and rotatable with said memher, and means communicating said chamber with a source of compressed air and including a centrally disposed air feed conduit with substantially radial passages connecting it to said chamber.

18. [n the apparatus defined in claim 1, said means defining said surface comprising a member having an air-permeable annular wall, said means for generating said air flow comprising means providing a first annular chamber secured around and rotatable with said member, and means communicating said chamber with a source of compressed air and including a further annular chamber with substantially axial passages connecting it to said first annular chamber.

19. The apparatus defined in claim 1, wherein an annular chamber surrounds said means defining said surface, means is provided for connecting said chamber to a source of compressed air, and said air flow is derived from said chamber.

20. An apparatus according to claim 1, wherein said surface is provided with apertures along one edge thereof forming discharge outlets for a separation fraction of the mixture of particulate materials.

21. An apparatus according to claim 1, wherein suction means is provided to remove a separated fraction from the interior of said surface. 

1. An apparatus for the separation of a mixture of particulate materials of different densities comprising means defining an air-permeable frusto-conical surface having a substantially vertical axis, means for feeding a mixture of said materials to said surface, means for rotating said surface about said axis for moving the mixture of particulate materials in a path defined by said air-permeable surface, means for generating an air flow substantially radially inward through said path for effective separation of said mixture into heavier and lighter fraction containing strata, and means for separately discharging said separated materials.
 2. An apparatus according to claim 1 having means to axially oscillate said surface.
 3. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and said mixture is disposed in a substantially annular layer at said surface and the means for generating said substantially radially inward air flow comprises a chamber surrounding said member externally of said surface and communicating with a source of compressed air, said chamber being secured to and rotatable with the said member.
 4. An apparatus according to claim 3 including means whereby compressed air is fed to said chamber surrounding the member through radial passages from a centrally disposed air feed conduit.
 5. An apparatus according to claim 3 including means whereby compressed air is fed to said chamber surrounding the member through axial passages from an annular chamber in communication with a source of compressed air.
 6. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and a discharge outlet for the heavier fractions is formed by circumferential apertures along one edge of said member.
 7. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and a discharge outlet for the lighter fractions communicates with suction means to remove separated lighter fractions from the interior of said member.
 8. An apparatus according to claim 1 wherein said surface is on the interior of a member mounted for rotation about said axis and first and second discharge outlets for the respective fractions are disposed at opposite axial ends of said member.
 9. An apparatus according to claim 1 wherein said surface is on the Interior of a member mounted for rotation about said axis and first and second discharge outlets for the respective fractions are disposed at the same axial end of said member.
 10. An apparatus according to claim 1 wherein said means for generating the air flow comprises means producing a pulsating air flow.
 11. An apparatus according to claim 10, wherein said means for producing a pulsating air flow comprises a mechanical interrupter disposed in an air feed duct communicating with a source of constant pressure air.
 12. Apparatus according to claim 1 wherein the air-permeable surface is formed by a plurality of spaced wedge wires.
 13. Apparatus according to claim 1 wherein the air-permeable surface is formed by expanded metal sheet.
 14. Apparatus according to claim 1 wherein the air-permeable surface is formed by sheet metal with punched apertures therein.
 15. Apparatus according to claim 1 wherein the air-permeable surface is formed by porous ceramic.
 16. Apparatus according to claim 1 wherein the air-permeable surface is formed by porous metal.
 17. In the apparatus defined in claim 1, said means defining said surface comprising a member having an air-permeable annular wall, said means for generating said air flow comprising means defining an annular chamber secured around and rotatable with said member, and means communicating said chamber with a source of compressed air and including a centrally disposed air feed conduit with substantially radial passages connecting it to said chamber.
 18. In the apparatus defined in claim 1, said means defining said surface comprising a member having an air-permeable annular wall, said means for generating said air flow comprising means providing a first annular chamber secured around and rotatable with said member, and means communicating said chamber with a source of compressed air and including a further annular chamber with substantially axial passages connecting it to said first annular chamber.
 19. The apparatus defined in claim 1, wherein an annular chamber surrounds said means defining said surface, means is provided for connecting said chamber to a source of compressed air, and said air flow is derived from said chamber.
 20. An apparatus according to claim 1, wherein said surface is provided with apertures along one edge thereof forming discharge outlets for a separation fraction of the mixture of particulate materials.
 21. An apparatus according to claim 1, wherein suction means is provided to remove a separated fraction from the interior of said surface. 